Various processes have been proposed for the production of high purity olefins by means of dehydration of alcohols, for example, a process in which ethylene is produced by dehydrating ethanol in the presence of solid acid catalysts such as alumina, silica, silica-alumina, zeolites, solid phospholic acid and the like (Japanese Patent Application Kokai No. 64-34929).
When dehydration is carried out using a solid acid as a catalyst, it is necessary to keep the reaction temperature at 250.degree. to 300.degree. C. or higher because of the great endothermic alcohol dehydration reaction. In addition, application of solid acid catalysts such as silica alumina, zeolites, solid phosphoric acid and the like is not desirable, because their strongly acidic nature causes decrease in the yield of olefins due to heavy materials formed by polymerization of the formed olefins, and the heavy material thus formed adheres no the catalyst surface and decreases the catalytic activity.
In such an instance, .gamma.-alumina may be used advantageously, because its weakly acidic nature does not entail polymerization and the like of formed olefins. However, the catalytic activity of .gamma.-alumina decreases greatly for a prolonged period of time. That is a significant drawback for using .gamma.-alumina to produce olefins.
It is known generally that 7-alumina is transformed into .alpha. form under high pressure or at a high temperature off 1000.degree. C. or more and that such a transition can be prevented by the addition of a metal oxide as a secondary component such as La.sub.2 O.sub.3, MgO, SiO.sub.2 or the like. However, it is difficult to apply such a prior arm means for the prevention of high temperature crystal phase transition to a reaction system at a low temperature of around 300.degree. C. under a low pressure, because nothing is known about the phenomenon or mechanism of crystal phase transition under such conditions.
Though it is possible to effect the hydration reaction without pressurization, such an operation requires complex handling because of the necessity to set additional process steps and is disadvantageous also from the economical point of view. That is, being gaseous under ambient temperature and pressure, lower olefins having 2 to 4 carbon atoms formed by the reaction must be liquefied prior to their purification by distillation or the like which requires cooling using a refrigerating machine or pressurization using a compressor.
In view of the above, it therefore becomes a primary object of the present invention to provide a process by which Lower olefins can be produced from lower alcohols with high yield and high selectivity for a prolonged period of time using simple equipments.